docs: def attached_sessions(self):
          """
          Return active :class:`Session` objects.
  
          This will not work where multiple tmux sessions are attached.
  
          Returns
          -------
          list of :class:`Session`
          """
  
          sessions = self._sessions
          attached_sessions = list()
  
          for session in sessions:
              if 'session_attached' in session:
                  # for now session_active is a unicode
                  if session.attached == '1':
                      logger.debug('session %s attached', session.name)
                      attached_sessions.append(session)
                  else:
                      continue
  
          return [Session(server=self, **s) for s in attached_sessions] or None